1. Field of the Invention
This invention relates to the arrangement of external terminals of a high power semiconductor device, such as an IGBT (insulated gate bipolar transistor) or a MOS FET, capable of high speed switching with high power and, more particularly, to the arrangement of module type semiconductor devices of high power capacity.
2. Description of the Related Art
IGBTs, representative of high speed and high power switching elements, have heretofore been made of module casings having a structure shown in FIGS. 1A and 1B. FIG. 1C shows an equivalent circuit of the IGBT.
In FIGS. 1A and 1B, a collector C and an emitter E the contacts to which serve as main current terminals, and a gate G and an emitter Es the contacts to which serve as input terminals for drive signals are used as corresponding, respectively, to a collector C and an emitter E the contacts to which serve as main current terminals in a bipolar transistor 101 shown in FIG. 1C, and a base B and an emitter Es the contacts to which serve as signal input terminals of same. Between the collector and the emitter is interposed a portion that functions similar to a diode, and hence a diode 102 is incorporated therebetween.
Within the module casing shown in FIGS. 1A and 1B are incorporated a plurality of, usually four, IGBT chips for being driven in parallel, thereby completing a high current semiconductor switching module.
The foregoing arrangement is disclosed in a publication entitled "1000 V 300 A Bipolar-Mode MOS FET (IGBT) Module", 1988 Proc. of ISPSD, pp 80-85, M. Hidesima, et al.
In some applications, a plurality of module type semiconductor devices of high power capacity of the arrangement shown in FIGS. 1A and 1B are connected in parallel to each other to obtain a circuit, such as an inverter, of further increased power capacity. In this case, it may be proposed to provide an ultra high power semiconductor device without connecting a plurality of modules but sealing several of modules in a single module casing. This, however, is not preferred on the following grounds.
First, the range of general applications of the module type semiconductor device becomes limited. Second, the number of parallel connections of IGBT chips within the module type semiconductor device become too excessive that the productivity of the device is decreased. Third, with an increase in a value of the current managed by a single module, wiring requirements within the device increase. Accordingly, use of module type semiconductor devices in parallel connections each having IGBT chips sealed within the casing in a convenient number of four is believed to be economical in high power capacity applications.
FIG. 2 is a circuit diagram showing the arrangement of a bridge-type chopper circuit using the module type semiconductor device of high capacity. Equivalent circuits Q1 to Q4 representing the IGBTs and each formed of a module type semiconductor device fabricated with a conventional module package shown in FIGS. 1A and 1B are connected two by two in parallel to obtain a high capacity. A half bridge structure showing the interconnection s of the equivalent circuits Q1 and Q2 in FIG. 2 is illustated in FIG. 3A. As shown, the module type semiconductor devices 301 and 302 are connected in parallel to form the equivalent circuit Q2, and the module type semiconductor devices 303 and 304 having equivalent properties as the devices 301 and 302 are connected in parallel to form the equivalent circuit Q1. A bus bar 305 connects main current terminals of the respective emitters of the module type semiconductor devices 301 and 302, and a bus bar 306 connects main current terminals of the respective collectors of the module type semiconductor devices 303 and 304. Further, a bus bar 307 makes interconnections between the collector main current terminals of the module type semiconductor devices 301 and 302, between the emitter main current terminals of the module type semiconductor devices 303 and 304, and between the module type semiconductor devices 301 and 302, and 303 and 304. The structure as viewed from an arrow 310 in FIG. 3A is shown in FIG. 3B.
As shown in FIGS. 3A and 3B, it is necessary to form openings 307a and 307b in the bus bar 307 at locations above signal input terminals Es and G of the module type semiconductor devices 303 and 304 to enable wiring connections to the signal terminals.
It has been found that, when the module type semiconductor devices fabricated with the module casing as shown in FIGS. 1A and 1B are subjected to bus bar wiring as shown in FIGS. 3A and 3B, a problem occurs that as the proposed current capacity of the module type semiconductor device increases and the proposed speed of switching operation increases, an inductance component of commutating circuits which increases surge voltage becomes substantial, requiring setting a large margin for the voltage withstandability of the circuit elements.
Moving to FIG. 2, the mechanism of commutating operation and the generation of surge voltage will be explained. The circuit shown is a so-called bridge circuit in which the IGBTs Q1 and Q2 are connected in series between a bus 202 and a bus 203 leading from a DC power supply 201, the IGBTs Q3 and Q4 are similarly connected in series therebetween, and an electric reactor 204 and a resistor 205 are connected between connecting points midway between the respective series connection lines.
Assume now that the IGBTs Q1 and Q4 are on and current is flowing through the circuit shown by arrows in solid lines. If the IGBT Q1 is turned off, a load current commutates through the circuit shown by an arrow in a dotted line. At this time, inductance L of the circuit and the rate of change of current di/dt generate a counter electromotive force of -L(di/dt) as surge voltage, which is impressed across the IGBT Q1 which has been turned off.
The inductance L here is affected by an inductance component of the bus 202 between nodes a and b, an inductance component of the bus 203 between nodes h and i, inductance components of the wiring between nodes b and c and between d and e, and by inductance components of the wiring between nodes e and f and between g and h.
If, for example, the inductance L is 0.2 .mu.H and an IGBT with a current of 400 A is turned off at time intervals of 0.3 .mu.s, the following equation holds. ##EQU1##
Equation (1) suggests the production of a surge voltage of approximately 270 V. To use an IGBT element at 500 V in the circuit arrangement shown in FIG. 2, it is necessary to use the voltage of the power supply at 230 V or below, so that use of IGBTs becomes limitative.
Where a module type semiconductor device of a package of the structure shown in FIGS. 1A and 1B is used, what has been done in an attempt to decrease inductance components as far as possible is to permit the collector C of the module forming Q1 and the emitter E of the module forming Q2 to be connected by a cupper plate and to punch out the cupper plate at portions of the terminals G and Es. This has resulted in lengthy wiring and limited surface area of the cupper plate and hence in an increase in commutating inductance. With increasing demand on still higher speed operation of module type semiconductor devices, a decrease in the commutating inductance has been sought.